As shown in FIG. 1, for example, a conventional optical semiconductor device mounted with an optical semiconductor element includes a metal lead frame including a first plate portion 1 and a second plate portion 2, an optical semiconductor element 3 provided on the metal lead frame, and a reflector 4 of a resin material provided around the optical semiconductor element 3 and filling a gap between the first plate portion 1 and the second plate portion 2 for reflecting light. The optical semiconductor element 3 is disposed in a recess 5 defined by the metal lead frame and an inner peripheral surface of the reflector 4, and is encapsulated with a transparent resin such as a silicone resin containing a fluorescent material, as required, whereby an encapsulation resin layer 6 is formed. In FIG. 1, reference numerals 7, 8 designate bonding wires which are provided as required for electrically connecting the metal lead frame to the optical semiconductor element 3.
In recent years, the reflector 4 is formed, for example, by transfer-molding a thermosetting resin typified by an epoxy resin for production of the optical semiconductor device. It is a conventional practice to blend titanium oxide (white pigment) with the thermosetting resin, so that the reflector 4 can reflect the light emitted from the optical semiconductor element 3 (see PTL1).
The formation of the reflector 4 by the transfer molding and the like causes a problem of, for example, warpage. For suppression of the warpage, it is necessary to blend a higher proportion of an inorganic filler with a molding material essentially containing the thermosetting resin. Where the molding material is prepared by using a higher proportion of the inorganic filler, however, an additional problem arises that the molding material has an increased viscosity.